The invention relates to proximity detectors of the analog inductive type comprising a parallel oscillating circuit the coefficient of quality factor and the inherent impedance of which vary as a function of the distance of a metal article which influences the coil and an amplifying means, the circuit being arranged to inject the output current of the amplifying means into said oscillating circuit through a resistance, said oscillating circuit forming part of a feedback circuit between the output and the input of the amplifying means.
These analog proximity detectors are capable of many industrial uses, such as the classifying of articles by their dimensions or by their position, measuring the speed of their approach, the indication of their direction of displacement, and others.
Such applications are suitable for distances which generally cannot exceed several centimeters and it would be desirable, at this scale of distances, to generate an analog signal which is as linear a function as possible of the distance.
The known circuits do not obtain a satisfactory linearity, in particular from the fact that they are fed at constant voltage and that the output signal is derived from the high frequency oscillation itself, which introduces a disturbance in the operation of the oscillatory circuit by applying an impedance at its terminals.